A semiconductor device used in a critical system, such as a space communication system, is required to reliably perform under severe environmental conditions for an extended period of time, for example, for a period of at least ten years or longer. Various environmental tests are performed in order to evaluate the quality of the semiconductor device under these conditions.
A high temperature environmental test of a microwave semiconductor device is performed at a temperature ranging from 200.degree. C. to 250.degree. C. for a predetermined period. The test is indispensable for developing new highly reliable semiconductor devices since the test is used to estimate the life of the new semiconductor device under a critical condition.
One problem that occurs with a high temperature environmental test is when a microwave semiconductor device, such as a gallium arsenide field effect transistor (GaAs-FET) which is operable at a frequency up to 18 GHz is tested. Generally, a holding device is used for holding the microwave semiconductor device which is placed into the testing apparatus. The holding device includes plates which have microstrip matching circuits. The holding device also has connectors which are connected to outer circuits arranged in the extreme proximity of the semiconductor device to be tested. The connectors are provided in order to reduce the microwave power reflection due to the sharp transition of the transmitting impedance of the transmission lines.
In the prior art high temperature environmental testing apparatus, the microstrip circuit plates and the connectors are exposed to a high temperature environment together with the semiconductor device under test. Thus, during the testing period, the microstrip matching circuits and the connectors are subject to some degree of damage such as oxidation of the contacts of the connectors and chemical and mechanical damage of the microstrip matching circuit plates. The chemical and mechanical damage includes damage done to microstrip patterns which are formed on substrates. The microstrip patterns are damaged due to cyclical thermal stress caused by intermittent breaks for measuring the change of the electric characteristics of the semiconductor device during the test period.
FIG. 1 is a perspective view of a prior art holding device used for a high temperature environmental test of a semiconductor device, such as a GaAs-FET 62, for example. Microstrip matching circuit plates 60 are provided on a base block 64 made of copper or aluminum. The microstrip matching circuit plates are connected to the semiconductor device to be tested. Each circuit plate 60 comprises a substrate 63 made of alumina, teflon, etc., a microstrip pattern 67 including a transmission line 68, formed on the top surface of the substrate 63, and a co-axial connector 66 connected to the transmission line 68. The holding device further includes a heat block 65 made of copper or aluminum, having a heating source therein.
The semiconductor device, the GaAs-FET 62, usually has a metallic base 69 and leads 61 for input and output of transmission power. The semiconductor device is electrically grounded through the metallic base 69. The FET 62 is placed into the high temperature testing apparatus on the holding device. This is done by tightly attaching the metallic base 69 to the base block 64 using screws 70. The leads 61 are soldered or pressed to the transmission lines 68. Then, the base block 64 is placed on the heat block 65 in contact face-to-face. With such a structural configuration, the heat generated in the heat block 65 is transferred to the base block 64, and then to the FET 62 through the metallic base 69 to raise the temperature of the FET 62 up to a predetermined testing value, such as 250.degree. C. However, at the same time, the heat is easily transferred to the microstrip matching circuit plates 60 and the co-axial connectors 66, raising the temperatures of these elements to a high level approximately near the testing temperature. As a result, the above-described damage occurs.
No problems arise when the ground potential is applied to the semiconductor device 62 and to the microstrip matching circuit plates 68 because the ground potential is applied to the base block 64.
The above-described defects of the prior art holding device need to be eliminated in order to achieve a more reliable high temperature environmental test result.